Heh I this is like oldies but goodies weekend, first formats - then CSS.
I have never used CSS before, but I have been writing more HTML this weekend than in the past year, so I decided to find out what this CSS was all about. This link was quite useful - but I think CSS is easily learned by example and by hand, so combined with a few google searches - I was up and running with my first style.css file...
w3schools has some nice reference and learning material on (X)HTML and CSS among other things.
Check 'em out.